As disclosed in French patent application number 2 668 246, such a combustion chamber normally comprises two coaxial walls of revolution that extend one inside the other and are connected to each other at one of their ends by an annular chamber bottom wall comprising air intake openings and fuel inlet means.
The inner wall and the outer wall form an annular bypass duct inside which air output by a high pressure compressor located on the upstream side of the combustion chamber circulates.
Conventionally, a fraction of this air supplies the combustion zone, axially through air intake openings formed at the bottom of the chamber and transversely through primary air injection holes perforated in the inner and outer walls of the chamber.
Furthermore, the inner and outer walls of this combustion chamber usually need to be cooled, due to the high temperatures inside the combustion chamber. To achieve this, current combustion chambers use well-known multi-perforation cooling processes. Multi-perforation consists of forming a large number of cooling air injection orifices in the walls of the combustion chamber. Therefore air passing through these orifices lowers the temperature of the walls and then the temperature of the combustion chamber.
These orifices are usually made by Laser drilling. Orifices 1 (shown in FIG. 1) passing through the wall 2 of a combustion chamber are inclined at angle αl equal to about 30 degrees from to a plane P tangent to the profile of the wall 2, in order to cool a larger area. Regardless of the drilling method used, the geometric cross-section of the tool (and therefore of the laser in the example shown) is always circular. Therefore the shape of the cross-section 3 of the outer end (in other words the end located at the outer surface of the wall) of each orifice 1 (see FIG. 2) obtained by inclined penetration of a tool with a circular cross-section into the profile of the wall 2 is elliptical.
Furthermore, during the operation of the turbine machine, the inner and outer walls of the combustion chamber expand thermally and heterogeneously and large vibrations occur in them, which causes high stresses at the edges of the orifices 1.
As mentioned above, a standard combustion chamber is perforated by a multitude of cooling orifices arranged in a staggered manner, and oriented along the same direction. In one embodiment, each outer end of an orifice has an elliptical cross-section, the major axis of which is approximately parallel to the axis of the combustion chamber. Thus, in a zone in which the highest stresses are perpendicular to the axis of the combustion chamber (and therefore to the major axis of the ellipse), the highest mechanical stresses are concentrated at the small radius r of the ellipse. These stresses eventually lead to the development of cracks or fissures at the edges of orifices 1, the cracks then propagating to the adjacent orifices 1 along the direction of the axis of the combustion chamber.
This feature severely limits the life of walls forming the combustion chamber.
Another standard combustion chamber comprises walls in which some orifices also have an elliptical shape but oriented along a different direction. For example, the large radius of the elliptical-shaped section is perpendicular to the axis of the combustion chamber. Thus, if the highest stresses in the zone in which such an orifice is located are perpendicular to the axis of the combustion chamber and therefore parallel to the major axis of the ellipse, the stresses at the small radius r of the ellipse will be lower. Such an embodiment can delay the appearance of cracks at the edge of each of the orifices, at the detriment of the airflow circulating in the combustion chamber.
The main disadvantage of the above mentioned embodiment is the fact that the airflow input into the combustion chamber through this multitude of orifices oriented in different directions is not homogeneous. These different orientations hinder the axial direction of the airflows and aerodynamic disturbances are created.